The present invention relates to a method and an apparatus for detecting an abnormality in an analog-to-digital converter, and more particularly to the method and the apparatus for detecting an abnormality which method and apparatus are suitable to an analog-to-digital converter required to produce a highly accuracy and reliable output.
To collect information about an environment represented by various analog values, in general, an analog-to-digital converter (referred to as an A/D converter) is now widely used.
The following description will be oriented to the general arrangement of the A/D converter. Plural analog signals inputted from the external are selected in an input selecting unit. The selected analog signal is added to an analog-to-digital (A/D) converting unit through a signal amplifying unit so that the selected analog signal may be converted into the digital signal in the A/D converting unit. Then, the digital signal is outputted at an output unit. The timing on which each of A/D converting operations is executed is determined by the control unit.
The A/D converter needs some kinds of voltages such as a reference voltage, a supply voltage, and a logic voltage for its operation.
This type of A/D converter has a disadvantage that if one or more of those necessary voltages are partially fluctuated, the digital signal converted from the corresponding analog signal may not be kept constant.
In the technique disclosed in JP-A-3-268610, in addition to the analog signal to be converted, the necessary voltage to the A/D converter such as a reference voltage is supplied to the input selecting unit. When the input selecting unit selects the reference voltage, the reference voltage is supplied from the input selecting unit to the A/D converting unit, in which it is converted to the corresponding digital value. Then, the difference between the converted digital value and a predetermined value is taken. If the difference is equal to or less than a predetermined allowable value, the correction is automatically executed. If the difference is more than the predetermined allowable value, it is determined that abnormality takes place.
The technique disclosed in JP-A-3-268610, however, operates to immediately determine that the A/D converter is abnormal even if a transient fault (for example, noises) comes from the outside. If the reference voltage or the supply voltage may be variable, the output of the A/D converter is less reliable, because the output is the converted digital value. Moreover, the technique disables to determine if the detected abnormality is transient or fatal. In the case of abnormality occurring in the supply voltage, it is fatal.